Laser emission device with integrated light modulator

ABSTRACT

Laser emission device with integrated light modulator comprising: a multilayer waveguide comprising, on a support layer, a first guiding layer, a first doped layer, a second guiding layer of light amplifying material, and a biasing second doped layer opposite the first doped layer, the waveguide comprising a laser amplification section ( 50 ), a light modulation section ( 52 ) comprising an extraction zone for radiating the light, a transition section ( 51 ) inserted between the laser amplification section and the light modulation section, a positive first electrode for injecting a pumping current into the laser amplification section, a positive second electrode for injecting a modulation signal into the modulation section, a negative third electrode, and a reference fourth electrode, the second doped layer comprising an electrical insulation situated in the transition section to form a resistive channel.

REFERENCE TO RELATED APPLICATION

The present application claims benefit to European Patent ApplicationNo. 13305845.3, filed Jun. 21, 2013, whose disclosure is herebyincorporated by reference in its entirety into the present disclosure.

TECHNICAL FIELD

The invention relates to the field of modulable semiconductor laseremission devices, notably to a laser source that can be used in opticalcommunications and in particular in an optical network with wavelengthdivision multiplexing (WDM).

TECHNOLOGICAL BACKGROUND

An electro-absorption modulator coupled to a laser source exists withthe use of a biasing tee making it possible to associate a DC componentwith the radio frequency signal. The biasing tee has a significant bulkwhich is prohibitive for the production of a compact component.Furthermore, this solution necessitates the use of a matching resistorwith a value of 50 ohms at the terminals of the electro-absorptionmodulator. This resistor produces a heat dissipation of 300 mW.

SUMMARY OF THE INVENTION

One idea on which the invention is based is to provide a laser deviceincluding an electro-absorption modulator of small bulk. Another idea onwhich the invention is based is to provide said device with reduced heatdissipation.

According to one embodiment, the invention provides a laser emissiondevice with integrated light modulator comprising:

a multilayer waveguide, the waveguide extending along a longitudinaldirection of the device,

the waveguide comprising, on a support layer made of silicon dioxide,

a first guiding layer of silicon,

a first doped layer,

a second guiding layer of light amplifying material, and

a biasing second doped layer opposite the first doped layer,

the waveguide comprising, along the longitudinal direction

a laser amplification section,

a light modulation section comprising an extraction zone for radiatingthe light of a resonant optical mode towards the exterior of the device,and a transition section inserted in the longitudinal direction, betweenthe laser amplification section and the light modulation section,the device also comprising:a positive first electrode coupled to the laser amplification section toinject a pumping current into the laser amplification section,a positive second electrode coupled to the light modulation section toinject a modulation signal into the modulation section,a negative third electrode coupled to the first doped layer in themodulation section to apply a biasing electrical potential to the firstdoped layer, and a reference fourth electrode coupled to the first dopedlayer in the laser amplification section,in which the second doped layer comprises an electrical insulationsituated in the transition section of the waveguide to form a resistivechannel.

According to embodiments, said laser emission device with integratedlight modulation can comprise one or more of the following features.

According to an embodiment, the electrical insulation is obtained by aprotonated insertion zone of the second doped layer.

By virtue of these features, the device remains compact.

According to an embodiment, the resistive channel exhibits a resistanceof between 1 and 10 kΩ, which makes it possible to independently set thebias of the laser and the bias of the modulator.

By virtue of these features, the dissipated power is limited, to anegligible level.

According to an embodiment, the second doped layer and the positivefirst electrode form a narrow ridge positioned on the second guidinglayer, the narrow ridge having a width approximately 5 times less thanthat of the second guiding layer in a part of the amplification section.

According to an embodiment, the width of the second guiding layer in apart of the amplification section and a part of the modulation section,called full width zones, is approximately 5 times greater than thewidth, called narrow zone, of said second guiding layer in thetransition section.

According to an embodiment, the width of the second guiding layer isprogressively reduced from the width of each full width zone to thewidth of the narrow zone.

By virtue of these features, the optical propagation of the mode betweenthe amplification section and the modulation section is not disturbed.

According to an embodiment, a width wise profile of the first guidinglayer is identical to a width wise profile of the second guiding layer.

By virtue of these features, the optical propagation of the mode in thewaveguide is not disturbed.

According to an embodiment, the first guiding layer, the first dopedlayer, the second guiding layer, the second doped layer and the positivefirst electrode form a narrow ridge in the transition section.

By virtue of these features, the transition section forms a resistivechannel of small bulk.

According to an embodiment, the resistive channel has a width less than3 μm.

According to an embodiment, the transition zone has a lengthsubstantially equal to 50 μm.

According to an embodiment, the second doped layer and the third dopedlayer are selected from materials from the group III-V ofsemiconductors.

By virtue of these features, the device can be in a configuration bondedonto silicon on insulator.

According to an embodiment, the second doped layer has a thickness lessthan 250 nm.

By virtue of these features, the production of a structure composed ofmaterials from the group III-V bonded onto silicon on insulator ispossible without disturbing the optical mode.

According to an embodiment, the positive first electrode and the seconddoped layer are separated by a layer of a positively doped galliumindium arsenide alloy.

According to an embodiment, the positive second electrode and the secondpassive layer of said modulation section are separated by a layer of apositively doped gallium indium arsenide alloy.

According to an embodiment, the negative third electrode is connected toa negative DC potential, the positive first electrode is connected to apositive DC potential, the reference electrode is connected to an earthelectrical potential and the positive second electrode is connected to abase band signal generator.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be better understood, and other aims, details,features and advantages thereof will become more clearly apparent fromthe following description of a particular embodiment of the invention,given purely as an illustrative and non-limiting example, with referenceto the attached drawings.

FIG. 1 is a longitudinal cross section of the slice of a laser device.

FIG. 2 is a plan view of the device of FIG. 1.

FIG. 3 is a cross-sectional view of the device of FIG. 1, along thecutting line III-III.

FIG. 4 is the view of a section of the device of FIG. 1 along thesection IV-IV.

FIG. 5 is a functional schematic representation of the device of FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to FIGS. 1 to 4, there now follows a description of asemiconductor laser source 1 represented from different views, notablycross-sectional views. FIGS. 1 and 2 represent the device, with alongitudinal enlargement over the transition section. By convention,vertical direction is used to denote the direction at right angles tothe stratification of the layers following one another in the slice of across-sectional view or a section of the component.

Starting from the bottom of the figures, in the vertical direction, thelaser source comprises a first semiconductor optical component 2,produced as silicon on insulator (SOI). This first optical componentconsists of a base layer 3 of silicon dioxide SiO2, and a guiding layer4, of silicon. The interfaces between layers are planar and parallel.The guiding layer 4 is covered with a thin layer of silicon dioxide, notrepresented.

On the top surface 5 of the component 2, there is arranged, by bonding,a semiconductor multilayer structure 6, produced for example inmaterials from the group III-V. This multilayer structure comprises anegatively-doped first layer 7, an active layer 8, two positively-dopedlayers 9 and 16. The doped layers 7 and 9 are doped differently, with anopposite bias to allow the application of an electrical potentialbetween the two layers. The assembly that is thus obtained forms a P-Njunction. Thus, the doped layer 7 is negatively doped, whereas the dopedlayer 8 is positively doped.

The duly assembled assembly of layers constitutes a waveguide forming aridge orientated along a longitudinal axis, which corresponds to theaxis I-I of the cross section of FIG. 1, represented in FIG. 2.

Along the longitudinal axis, the device comprises an amplificationsection 10 followed by a transition section 11, then a modulationsection 12.

The waveguide in the amplification section 10 contains an amplificationmeans corresponding to the active layer 8. This amplification means isobtained using a quantum-well material. This section produces astimulated light emission which is propagated through the transitionsection 11, to the modulation section 12. The amplification section 10is powered using two electrodes, making it possible to apply anelectrical signal to the P-N junction. A positive first electrode 17 isproduced using a metallization on a part of the doped layer 9. Thispositive electrode 17 begins in the amplification section 10 and extendsinto a part of the transition section 11. This positive electrode 17leaves a non-metallized portion 22 of the doped layer 9. A referenceelectrode 15 is also produced by metallization, on a portion of thedoped layer 7. To improve the biasing, a strongly positively-doped layer16 is sandwiched between the positively doped layer 9 and the electrode17.

FIGS. 2 and 3 show also that the width of different layers of materialIII-V forming the ridge varies, in the amplification section. The width19 of the active layer 8 is, for example, five times greater than thewidth 20 of the top layers, the doped layers 9 and 16 and the positiveelectrode 17. This ridge profile 30 is known as “shallow-ridge”structure. In the interface zone between the amplification section 10and the transition section 11, the width of the active layer 8 decreasesin a bevel to form an active layer of the width 20 of the top layers inthe transition section 11.

In the transition section, as FIG. 4 shows, the waveguide forms a deepridge 40, known as deep-ridge structure; this deep ridge 40 is narrow.It comprises, at one end, at the interface with the modulation section,the non-metallized portion 22. In this non-metallized portion 22, thedoped layer 9 is insulated in the depth 23. This insulation 23 consistsof a proton-charged zone. This charge is obtained by a high-energyhydrogen bombardment which penetrates the doped layer 9.

The length 21 of the transition section is relatively great relative tothe width 20 of the ridge 40. The resistance of a duly constructed Nchannel is the product of the resistance squared of this layer,multiplied by the length 21 of the channel, divided by its width 20.

At the second end of the transition section, at the interface of theinsulated zone 23 and of the modulation section 12, the active layer 8covers a width similar to its width 19 in the amplification section 10.The ridge thus formed in the modulation section 12 is called“shallow-ridge” structure, just like for the amplification section 10.

The modulator also comprises two electrodes. These electrodes allow forthe biasing and therefore the control of the modulator. They are used toapply an electrical field in a direction at right angles to themodulated light beam. A negative electrode 24 is coupled to the dopedlayer 7, to apply a biasing electrical potential to the doped layer 7.For example, a DC power supply of −4V is applied to this negativeelectrode 24. A positive electrode 26 is coupled via a stronglypositively-doped layer 25, to the part of the doped layer 9 situated inthe modulation section 12. The positive electrode 26 makes it possibleto inject a modulation signal.

Throughout the device, the width profile of the guiding layer 4 isidentical to that of the active layer 8. Furthermore, the change ofstructure between the three sections of the optical component andtherefore the transition from a shallow-ridge structure to a deep-ridgestructure and then again to a shallow-ridge structure does not disturbthe optical propagation of the mode in the component.

Such a component can for example be obtained using a doped layer 7 madeof indium phosphide (InP). The active layer 8 is obtained using aquantum-well material, for example a structure consisting of 6 InGaAsPwells and barriers emitting at a wavelength of 1.55 μm. The doped layer9 is for example made of indium phosphide (InP). The layers 16 and 25are produced using a gallium indium arsenide alloy (InGaAs). Theelectrodes 15, 17, 24 and 26 are produced in a material with lowelectrical resistance. The material used can also have a high resistanceto oxidation. For example, the electrodes are produced with gold (Au).

With reference to FIG. 5, the biasing principle of the laser emissiondevice with integrated light modulator is schematically represented. Inthis system, laser amplification section 50 behaves like a diode whosenegative electrode is linked to the earth 60, and the positive electrodeis linked to a DC voltage 62. The DC voltage 62 applied is, for example+2 volts. The transition section 51 separates the amplification section50 from the modulation section 52. This modulation section 52 is linkedby its negative electrode to a biasing electrical potential 64. Thisbiasing electrical potential 64 is, for example −4 volts. A modulationsignal 61 is applied to the positive electrode of the modulation section52 to control the emission of the laser device.

The structure of the laser emission device described in FIGS. 1 to 5makes it possible to create a resistive channel of at least one 1 kΩ inthe transition section 51, 11, between the negatively doped layer 7 ofthe amplification section and the negatively doped layer 7 of themodulation section.

For example, for a doped layer 7, with a doping of 7 10¹⁷ cm-3, of whichthe thickness is 200 nm, the width 20 is 2 μm, the length 21 is 50 μmfor a laser device with a length of at least 300 μm, the resistance persquare is 100 ohm. The inter-electrode resistance in the transitionsection reaches 2.5 Ωk.

Although the invention has been described in conjunction with aparticular embodiment, it is obvious that it is in no way limitedthereto and that it comprises all the technical equivalents of the meansdescribed as well as their combinations provided that they fall withinthe framework of the invention.

In operation, the electrode 26 is powered by a base band signalgenerator, not represented, which can be produced in different forms, ina unitary or distributed manner, by means of hardware and/or softwarecomponents. Hardware components that can be used are custom integratedcircuits ASICs, programmable logic arrays FPGAs or microprocessors.Software components can be written in different programming languages,for example C. C++, Java or VHDL. This list is not exhaustive.

“Comprise” or “include” and its conjugated forms does not preclude thepresence of elements or steps other than those described in a claim. Theuse of the indefinite article “a” or “an” for an element or a step doesnot preclude, unless stated otherwise, the presence of a plurality ofsuch elements or steps.

In the claims, any reference symbol between parentheses should not beinterpreted as a limitation of the claim.

The invention claimed is:
 1. Laser emission device with integrated lightmodulator comprising: a multilayer waveguide, the waveguide extendingalong a longitudinal direction of the device, the waveguide comprising,on a support layer made of silicon dioxide, a first guiding layer ofsilicon, a first doped layer, a second guiding layer of light amplifyingmaterial, and a biasing second doped layer opposite the first dopedlayer, the waveguide comprising, along the longitudinal direction alaser amplification section, a light modulation section comprising anextraction zone for radiating the light of a resonant optical modetowards the exterior of the device, and a transition section inserted inthe longitudinal direction, between the laser amplification section andthe light modulation section, the device also comprising: a positivefirst electrode coupled to the laser amplification section, and apositive DC potential connected to the positive first electrode toinject a pumping current into the laser amplification section, whereinthe device also comprises: a positive second electrode coupled to thelight modulation section, a base band signal generator connected to thepositive second electrode to inject a modulation signal into themodulation section, a negative third electrode coupled to the firstdoped layer in the modulation section to apply a biasing electricalpotential to the first doped layer, the modulation signal being appliedbetween the positive second electrode and the negative third electrodeto control the emission of the laser device, a reference fourthelectrode coupled to the first doped layer in the laser amplificationsection, an earth electrical potential linked to the reference fourthelectrode, the pumping current being injected by applying a DC voltagebetween the fourth reference electrode and the positive first electrode,wherein the first guiding layer, the first doped layer, the secondguiding layer, the second doped layer and the positive first electrodeform a narrow ridge in the transition section, the narrow ridge beingnarrower than the first doped layer in the laser amplification sectionand the light modulation section, such that the first doped layer formsa first resistive channel of small bulk between the third electrode andthe fourth electrode in the transition section, wherein the second dopedlayer comprises an electrical insulation situated in the transitionsection of the waveguide to form a second resistive channel between thefirst electrode and the second electrode in the second doped layer, inwhich the electrical insulation is obtained by a protonated insertionzone of the second doped layer, and the first resistive channel exhibitsa resistance of between 1 and 10 kΩ, which makes it possible toindependently apply the DC voltage to the laser amplification sectionand the modulation signal to the light modulation section.
 2. Deviceaccording to claim 1, wherein the second doped layer and the positivefirst electrode in the narrow ridge have a width approximately 5 timesless than that of the second guiding layer in a part of theamplification section.
 3. Device according to claim 1, wherein the widthof the second guiding layer in a part of the amplification section and apart of the modulation section, called full width zones, isapproximately 5 times greater than the width, called narrow zone, ofsaid second guiding layer in the transition section.
 4. Device accordingto claim 1, wherein the width of the second guiding layer isprogressively reduced from the width of each full width zone to thewidth of the narrow zone.
 5. Device according to claim 1, wherein awidth wise profile of the first guiding layer is identical to a widthwise profile of the second guiding layer.
 6. Device according to claim1, wherein the narrow ridge has a width less than 3 μm.
 7. Deviceaccording to claim 1, wherein the transition zone has a lengthsubstantially equal to 50 μm.
 8. Device according to claim 1, whereinthe second doped layer and the third doped layer are selected frommaterials from the group III-V of semiconductors.
 9. Device according toclaim 1, wherein the second doped layer has a thickness less than 250nm.
 10. Device according to claim 1, wherein the positive firstelectrode and the second doped layer of said amplification section areseparated by a layer of a positively doped gallium indium arsenidealloy.
 11. Device according to claim 1, wherein the positive secondelectrode and the second doped layer of said modulation section areseparated by a layer of a positively doped gallium indium arsenidealloy.